The present invention relates to an artificial fusion implant to be placed into the intervertebral space left after the removal of a damaged spinal disc.
The purpose of the present invention is to provide an implant to be placed within the intervertebral disc space and provide for the permanent elimination of all motion at that location. To do so, the device is space occupying within the disc space, rigid, self-stabilizing to resist dislodgement, stabilizing to the adjacent spinal vertebrae to eliminate local motion, and able to intrinsically participate in a vertebra to vertebra bony fusion so as to assure the permanency of the result.
At present, following the removal of a damaged disc, either bone or nothing is placed into the space left. Placing nothing in the space allows the space to collapse which nay result in damage to the nerves; or the space may fill with scar tissue and eventually lead to a reherniation. The use of bone is less than optimal in that the bone obtained from the patient requires additional surgery and is of limited availability in its most useful form, and if obtained elsewhere, lacks living bone cells, carries a significant risk of infection, and is also limited in supply as it is usually obtained from accident victims. Furthermore, regardless of the source of the bone, it is only marginal structurally and lacks a means to either stabilize itself against dislodgement, or to stabilize the adjacent vertebrae.
A review of related prior art will demonstrate the novelty of the present invention.
There have been an extensive number of attempts to develop an acceptable disc prothesis (an artificial disc). Such devices by design would be used to replace a damaged disc and seek to restore the height of the interspace and to restore the normal motion of that spinal joint. No such device has been found that is medically acceptable. This group of prosthetic or artificial disc replacements, seeking to preserve spinal motion and so are different from the present invention, would include:
U.S. Pat. No. 3,867,728 STUBSTAD--describing a flexible disc implant.
U.S. Pat. No. 4,349,921 KUNTZ--describing a flexible disc replacement with file like surface projections to discourage device dislocation.
U.S. Pat. No. 4,309,777 PATIL--describing a motion preserving implant with spiked outer surfaces to resist dislocation and containing a series of springs to urge the vertebrae away from each other.
U.S. Pat. No. 3,875,595 FRONING--describing a motion preserving bladder like disc replacement with two opposed stud-like projections to resist dislocation.
U.S. Pat. No. 2,372,622 FASSIO (FRENCH)--describing a emotion preserving implant comprising complimentary opposed convex and concave surfaces.
In summary then, these devices resemble the present invention only in that they are placed within the intervertebral space following the removal of a damaged disc. In that they seek to preserve spinal motion, they are diametrically different from the present invention which seeks to permanently eliminate all motion at that spinal segment.
A second related area of prior art includes those devices utilized to replace essentially wholly removed vertebra. Such removal is generally necessitated by extensive vertebral fractures, or tumors, and is not associated with the treatment of disc disease. While the present invention is to be placed within the disc space, these other vertebral devices cannot be placed within the disc space as at least one vertebra has already been removed such that there no longer remains a "disc space." Furthermore, these devices are limited in that they seek to perform as temporary structural members mechanically replacing the removed vertebra (not a removed disc), and do not intrinsically participate in supplying osteogenic material to achieve cross vertebrae bony fusion. Therefore, again unlike the present invention which provides for a source of osteogenesis, use of this group of devices must be accompanied by a further surgery consisting of a bone fusion procedure utilizing conventional technique. This group consisting of vertebral struts rather than disc replacements would include the following:
U.S. Pat. No. 4,553,273 WU--describing a turnbuckle like vertebral strut.
U.S. Pat. No. 4,401,112 REZAIAN--describing a turnbuckle like vertebral strut with the addition of a long stabilizing staple that spans the missing vertebral body.
U.S. Pat. No. 4,554,914 KAPP--describing a large distractible spike that elongates with a screw mechanism to span the gap left by the removal of a entire vertebra and to serve as an anchor for acrylic cement which is then used to replace the missing bone (vertebra).
U.S. Pat. No. 4,636,217 OGILVIE--describing a vertebral strut mechanism that can be implanted after at least one vertebra has been removed and which device consists of a mechanism for causing the engagement of screws into the vertebra above and the vertebra below the one removed.
In summary then, this group of devices differs from the present invention in that they are vertebra replacements struts, do not intrinsically participate in the bony fusion, can only be inserted in the limited circumstances where an entire vertebra has been removed from the anterior approach, and are not designed for, or intended to be used for the treatment of disc disease.
A third area of prior art related to the present invention includes all devices designed to be applied to one of the surfaces of the spine. Such devices include all types of plates, struts, and rods which are attached by hooks, wires and screws. These devices differ significantly from the present invention in that they are not inserted within the disc space, and furthermore do not intrinsically participate in supplying osteogenic material for the fusion.
Therefore, with these devices where permanent spinal immobilization is desired an additional surgery consisting of a spinal fusion performed by conventional means or the use of supplemental methylmethacrylate cement is required. Such devices applied to the spine, but not within the disc space, would include the following:
U.S. Pat. No. 4,604,995--STEPHENS--describing a "U" shaped metal rod attached to the posterior elements of the spine with wires to stabilize the spine over a large number of segments.
U.S. Pat. No. 2,677,369--KNOWLES--describing a metal column device to be placed posteriorly along the lumbar spine to be held in position by its shape alone and to block pressure across the posterior portions of the spinal column by locking the spine in full flexion thereby shifting the maximum weight back onto the patient's own disc.
Other devices are simply variations on the use of rods (e.g. Harrington Luque, Cotrel-Dubosset, Zielke), wires or cables (Dwyer), plates and screws (Steffee), or struts (Dunn, Knowles).
In summary, none of these devices are designed or can be used within the disc space, do not replace a damaged disc, and do not intrinsically participate in the generation of a bony fusion.
Another area of related prior art to be considered is that of devices designed to be placed within the vertebral interspace following the removal of a damaged disc, and seeking to eliminate further motion at that location.
Such a device is contained in U.S. Pat. No. 4,501,269 BAGBY--describing an implantable device and limited instrumentation. The method employed is as follows: a hole is bored transversely across the joint and then a hollow metal basket of larger diameter than the hole is then pounded into the hole and then filled with the bone debris generated by the drilling.
While the present invention (device, instrumentation, and method) may appear to bear some superficial resemblance Lo the BAGBY invention, it is minimal, while the differences are many fold and highly significant. These differences include the following:
1. Safety
The present invention provides for a system of completely guarded instrumentation so that all contiguous vital structures (e.g. large blood vessels, neural structures) are absolutely protected. Said instrumentation also makes overpenetration by the drill impossible. Such overpenetration in the cervical spine, for example, would result in the total paralysis or death of the patient. In the thoracic spine, the result would be complete paraplegia. In the lumbar spine, the result would be paraplegia or a life-threatening perforation of the aorta, vena cava, or iliac vessels. The present invention is atraumatically screwed into place while the BAGBY device, in contradistinction, is pounded into position. BAGBY describes that the implant is significantly larger in size than the hole drilled and must be pounded in. This is extremely dangerous and the pounding occurs directly over the spinal cord which is precariously vulnerable to percussive injury. Furthermore, while it is possible, for example in the lumbar spine, to insert the present invention away from the spinal cord and nerves, the BAGBY device must always be pounded directly towards the spinal cord.
Furthermore, since the BAGBY device is pounded into a smooth hole under great resistance, and lacking any specific design features to secure it, the device is highly susceptible to forceful ejection which would result in great danger to the patient and a clinical failure. The present invention, in contradistinction, is securely screwed into place, and possesses highly specialized locking threads to make accidental dislodgement impossible. Because of the proximity of the spinal cord, spinal nerves, and blood vessels, any implant dislodgement as might occur with the BAGBY device might have catastrophic consequences.
2. Broad Applicability
The BAGBY device can only be inserted from the front of the vertebral column, however, the present invention can be utilized in the cervical, thoracic, and lumbar spine, and can be inserted from behind (posteriorly) in the lumbar spine. This is of great importance in that the purpose of these devices is in the treatment of disc disease and probably greater than 99 percent of all lumbar operations for the treatment of disc disease are performed from behind where the present invention can easily be utilized, but the BAGBY device, as per his description, cannot.
3. Disc Removal
The BAGBY invention requires the complete removal of the disc prior to the drilling step, whereas the present invention eliminates the laborious separate process of disc removal and efficiently removes the disc and prepares the vertebral end plates in a single step.
4. Time Required
The present invention saves time over the BAGBY invention in that time is not wasted laboring to remove the disc prior to initiating the fusion. Also, since with the present invention the procedure is performed through a system of guarded instrumentation, time is not wasted constantly placing and replacing various soft tissue retractors throughout the procedure.
5. Implant Stability
Dislodgement of the implant would be a major source of device failure (an unsuccessful clinical result), and might result in patient paralysis or even death. As discussed, the BAGBY device lacks any specific means of achieving stability and since it is pounded in against resistance to achieve vertebral distraction, it is susceptible to forceful dislodgement by the tendency of the two distracted vertebrae, to return to their original positions squeezing out the device. The present invention however is screwed into place. As there is no unscrewing force present between the vertebrae and compression alone cannot dislodge the implant, the implant is inherently stable by its design. Furthermore, the threads of the present invention are highly specialized in that they are periodically interrupted such that the tail ends of each of the Labs so formed are blunted and twisted so as to resist accidental unscrewing. The removal of an implant with such "locking threads" requires the use of a special extractor included within the instrumentation. The stability of the present invention is still further enhanced, again in contradistinction to the BAGBY device, by the presence of a "bone ingrowth" surface texturing, which both increases the friction of the fit and allows for the direct growth of the vertebral bone into the casing of the implant itself.
6. Spinal Stability
The present invention is not only self-stabilizing, it also provides stability to the adjacent vertebrae in at least three ways that the BAGBY device cannot. First, the BAGBY device is placed transversely across the joint in the center, leaving both vertebrae free to rock back and forth over this round barrel shaped axis, much like a board over a barrel, being used for a seesaw.
Secondly, as the BAGBY device lacks any specific design features to resist sliding, it may actually behave as a third body allowing the translation of the vertebrae relative to the device and to each other.
Thirdly, any device can only provide stability if it remains properly seated. The present invention is inherently stable, and therefore assures that it will stabilize the adjacent vertebrae; rather than, as with the BAGBY device, where the instability of the spine to be treated may instead cause a dislocation of the implant, with further loss of spinal stability.
7. The Collapse of the Interspace
While both the present invention and the BAGBY device can be fabricated to withstand the compression forces within the interspace, the interspace may nevertheless collapse under the superincumbent body weight as the implant settles into the vertebral bone. This is related to the load per unit area. Again the present invention is superior to the BAGBY device in at least four ways. First, the present invention offers considerably greater surface area to distribute the load. Secondly, while the BAGBY device is placed centrally, the present device is placed bilaterally where the bone tends to be more cortical and much stronger out towards the rim. Thirdly, the present invention supports the load achieving an "I" beam effect, whereas the BAGBY implant does not. Fourthly, it is not pressure alone that causes the collapse of the bone adjacent to the implant, but also bony erosion that is caused by the motion under pressure of the implant against the bone. As discussed in item #6 above, the present invention alone is highly resistant to such motion, again diminishing the likelihood of erosion and interspace collapse.
8. Bone Ingrowth Surface Texturing
The present invention has a surface treatment of known and conventional technology to induce the growth of bone from the vertebrae directly into the casing material of the implant itself. The BAGBY device has no similar feature.
9. Fusion Mass
The BAGBY invention calls for removing the disc and then drilling a hole between the adjacent vertebrae. The bony debris so generated is then put into tie device. The present invention takes a core of pure bone producing marrow from the iliac crest, and then by use of a special press forcibly injects the device with an extremely dense compressed core of that osteogenic material until the material itself virtually extrudes from every cell of the implant.
10. The Probability of Achieving Fusion
The fusion rate within the spine is known to be related directly to the amount of exposed vascular bone bed area, the quality and quantity of the fusion mass available, and the extent of the stabilization obtained with all other factors being held constant. It would then be anticipated, that the fusion rate would be superior with the present invention as compared to the BAGBY device, because of optimal implant stability (#5), optimal spinal stability (#6), bone ingrowth surface treatment (#8), superior fusion mass (#9), and the greater exposed vertebral bony surface area (#7).
The last area of prior art possibly related to the present invention and therefore, to be considered related to "BONY INGROWTH", and patents either describe methods of producing materials and or materials or devices to achieve the same. Such patents would include:
U.S. Pat. No. 4,636,526 (DORMAN), No. 4,634,720 (DORMAN), No. 4,542,539 (ROWE), No. 4,405,319 (COSENTINO) No. 4,439,152 (SMALL), No. 4,168,326 (BROEMER), No. 4,535,485 (ASHMAN), No. 3,987,499 (SCHARBACH), No. 3,605,123 (HAHN), No. 4,655,777 (DUNN), No. 4,645,503 (LIN), No. 4,547,390 (ASHMAN), No. 4,608,052 (VAN KAMPEN), No. 4,698,375 (DORMAN), No. 4,661,536 (DORMAN), No. 3,952,334 (BOKROS), No. 3,905,047 (LONG), No. 4,693,721 (DUCHEYNE), No. 4,070,514 (ENTHERLY).
However, while the present invention would utilize bone ingrowth technology, it would do so with conventional technology.
In summary then, the present invention, instrumentation, and method, alone provides for a one stage discectomy, fusion, and interbody internal spinal fixation; that being performed more quickly, with greater safety, and more affectively, for all of the aforementioned reasons than is possible with any other known art.